Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of leaks that could release fuel vapors to the atmosphere. Evaporative leaks may be identified using engine-off natural vacuum (EONV) during conditions when a vehicle engine is not operating. In particular, a fuel system may be isolated at an engine-off event. The pressure in such a fuel system will increase if the tank is heated further (e.g., from hot exhaust or a hot parking surface) as liquid fuel vaporizes. As a fuel tank cools down, a vacuum is generated therein as fuel vapors condense to liquid fuel. Vacuum generation is monitored and leaks identified based on expected vacuum development or expected rates of vacuum development.
In order to preserve battery charge, a typical EONV test is subject to a time limit. A failure to reach a pressure or vacuum threshold before the end of the time limit may result in degradation being indicated, even if the fuel system is intact. The pressure rise portion of the test may execute until the fuel tank pressure curve reaches a zero-slope. If the pressure rise has a relatively low rate of constant increase (e.g., due to cool ambient conditions counteracting the pressure increase), and a significant amount of the time limit elapses prior to a zero-slope moment, the subsequent vacuum test may fail based on the limited amount of time remaining, regardless of the state of the fuel system.
Further, the entry conditions and thresholds for a typical EONV test are based on an inferred total amount of heat rejected into the fuel tank during the prior drive cycle. The inferred amount of heat may be based on engine run-time, integrated mass air flow, etc. However, the timing of heat energy transfer to the fuel tank significantly effects the fuel tank temperature at the initiation of the EONV test. A period of high-speed driving followed by a period of idling would indicate a high total amount of heat rejected, but much of the heat would dissipate from the tank during the idling period.
The inventors herein have recognized the above issues, and have developed systems and methods to at least partially address them. In one example, a method is provided, comprising terminating a pressure rise portion of an engine-off natural vacuum test based on an initial rate of change of a fuel system pressure upon sealing a fuel system; and initiating a vacuum portion of the engine-off natural vacuum test responsive to suspending the pressure rise portion. The initial rate of change may indicate a likelihood of the pressure rise portion reaching a pressure rise threshold. In this way, the vacuum portion of the test may be initiated earlier, increasing the likelihood of a conclusive result being obtained during a test time limit.
In another example, a method is provided, comprising: adjusting an evaporative emissions leak test parameter based on a time-weighted driving aggressiveness index; and indicating degradation based on the adjusted parameter. The time-weighted driving aggressiveness may provide a more accurate depiction of the heat rejected to the fuel tank at the point of initiating the evaporative emissions leak test. In this way, the leak test parameters may be more indicative of the current operating conditions, decreasing the likelihood of false failures. The adjusted parameters may include a pressure rise threshold and/or a vacuum threshold. In this way, the expected pressure change may reflect the amount of heat rejected to the fuel tank during the previous drive cycle. In some examples, the amount of heat rejected to the fuel tank may additionally or alternatively be based on an exhaust system temperature. The exhaust system temperature may be based on a resistance of a heating element coupled within a heated exhaust gas oxygen sensor. In this way, a liquid fuel temperature may be inferred without requiring a dedicated fuel temperature sensor or exhaust temperature sensor.